The only K-ras mutation detected was at codon 12 (GGT to GAT), and was identified in half of the neoplasms from exposure concentrations of 312, 625 and 1,250 ppm BMP and in feed controls. Our study suggests that mutations in the K-ras gene are involved in B6C3F1 lung carcinogenesis following BMP exposure. Based on the detection of the K-ras mutation, and the recent identification of a major lung susceptibility locus mapped to the distal part of chromosome 6 in the region of the K-ras gene, we are examining the lung tumors for loss of heterozygosity (LOH) on distal chromosome 6. To date allelic imbalances on chromosome 6 have been detected in BPM induced lung neoplasms.